This is a common upgrade issue with the built in web server.

This only applies if you are using the built in web server, not IIS.

In the elevated command prompt, run:

netsh http add urlacl url=http://*:<<port>>/ user=<<user>> 

where <<port>> is the port you have set the SQL Monitor web service to use, and <<user>> is the user the web service is running as.

You will then need to restart the SQL Monitor BaseMonitor Service, and the SQL Monitor Web Service.